"Each step increases the inferential distance."

Inferential distance means

how many reasoning steps separate your conclusion from directly observed evidence.

Example 1

Document

A factory employed 500 workers.

Conclusion

The factory had workers.

Very short inferential distance.

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Example 2

Factory records show rapid hiring after legal reform.

Conclusion

Legal reform probably increased labor mobility.

Moderate inferential distance.

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Example 3

Therefore the reformers secretly intended to create capitalism.

Now several additional reasoning steps have been added.

The inferential distance is much greater.

That is why historians expect correspondingly stronger evidence.